Ruby: Handle arbitrary method names

I remember watching a ruby screencast from Dave Thomas about using metaprogramming to create method names that do not need to adhere to identifier syntax requirements. Typically, an identifier must not contain spaces. And he showed how to create methods with spaces. But I don't remember how it's done.

I have arbitrary field names for a class that

include

s Mongoid::Document. This works fine, where _field can be a string like 'Hello World':